Display monitors such as cathode ray tubes (CRT's) and others such as liquid crystal displays (LCD's) and the like are known to exhibit a non-linear tone response to input signals that results in output luminances that are nonlinearly related to the digital gray level input signals. This non-linear tone response can be most simply expressed as a power law equation:L=Dγ  Eqn 1Where L is the output luminance exhibited by the display, D is the gray level digital input value (or gray level) driving the display, and the power-law γ is referred to as the “gamma” of the display. For CRT display monitors, gamma is typically in the range of 1.8 to 2.55. The tone response is graphically illustrated in FIG. 1 for a gamma of 2.5 and an 8 bit-depth luminance range. The response may additionally be influenced by components in the image path such as video look-up tables (LUTs) in graphic display adapters. In FIG. 1, it can be seen that for an input digital gray level of 50%, (binary value 128), the display monitor will provide an output luminance of only 18% (binary value 45). Beside a strong difference in image luminance, this non-linear response also causes a color shift in the displayed image relative to the input image data. For display monitors other than CRT's, such as liquid crystal displays (LCD's) and the like, some non-linear response can also be exhibited with similar undesired results.
With the foregoing in mind, it is deemed necessary to calibrate a display monitor so that the output intensities and colors correspond as closely as possible to the actual digital gray level input values. For a printer manufacturer, monitor calibration is particularly important because the final output print will almost always be compared to the impression the user had on the display monitor. In professional graphic arts environments, careful measurements and calibrations are performed for a display monitor, using specialized instruments. This is obviously not possible for a typical home or office user printing photographs, web pages, etc. Instead, various visual calibration techniques have been devised and implemented by which a user can perform a visual matching task, comparing patches having differing color and/or luminance levels in order to derive various characteristics of the display, include the gamma of the display. The monitor can thus be calibrated accordingly, to the extent possible, to exhibit a linear response to the input signals. The actual power law equation for a CRT display monitor is more complex than described in Eqn. 1 and can be modeled as:L=(αD+β)γ+φ  Eqn 2where: L=output luminance; D=input digital gray level; α=gain; β=offset; φ=external light flare; and γ=the display gamma as described above. Note that by setting β=φ=0 and α=1, Eqn (2) reduces to the simpler model shown in Eqn (1). It has been found that determining the gamma (γ) for a particular display monitor and compensating for same using the simpler model in Eqn (1) provides a satisfactory monitor calibration technique for certain applications. Monitor calibration based solely on gamma (γ) as in Eqn (1) is commonly referred to as “gamma correction” and is intended to adjust the display monitor to compensate for the gamma value so that the display monitor exhibits a linear response (γ=1).
It should be noted from FIG. 1 that the display monitor gamma has no effect on an input gray level of 0%, i.e., black, or on an input gray level of 100%, i.e., white, but does affect the continuous tone values between these two extremes. Known gamma correction techniques have attempted to exploit this phenomenon to provide a visual calibration technique as shown in FIG. 2. A visual matching pattern MP is displayed to the user on a monitor. The visual matching pattern MP comprises a plurality of test patches TP, each of which includes a half-tone portion HT of a known luminance, e.g., 50% half-tone screen, and a continuous tone portion CT each related to a known digital gray level. As is well know in the art, the half-tone portion HT of each test patch TP is defined by a grid or other pattern of binary intensities of 0% (“off”) or 100% which, at a typical viewing distance, is averaged by a human eye to a perceived continuous tone luminance level. Because the gamma value for a display monitor has no effect on the binary intensities of the half-tone portion HT, it provides a known reference luminance level depending upon the particular half-tone pattern used, and the half-tone portion HT is identical for each test patch TP. As shown, the half-tone pattern HT is intended to be perceived by a human as a 50% luminance or gray level. The continuous tone portions CT of the test patches TP correspond to respective digital gray levels that vary along a range that brackets the known luminance of the half-tone patterns HT, i.e., above, below and equal to the half-tone luminance. The user is then instructed to choose the test patch TP where the luminance of the continuous tone pattern CT most closely matches the luminance of the half-tone pattern HT. This selection process relates the digital gray level of the selected test patch TP (e.g., 70%) to the known luminance of the half-tone pattern HT (e.g., 50%) which allows the gamma value for the display to be derived. Often, this visual luminance matching operation is performed for each color separation, e.g., red, green, blue, of the display monitor and can be performed for more than one half-tone pattern HT.
This matching operation and others have been found to be deficient for a variety of reasons. The task of matching two colors or gray levels seems appealingly easy and simple, at least to one of ordinary skill in the art. For unskilled/untrained users, the task can be very difficult for various reasons. The user is asked to match patterns that have an inherently different spatial structure, and is forced to abstract the half-tone pattern to a luminance level in order to make the comparison. The difficulty is compounded by the fact that luminance/color matching is not a task that is commonly performed by users on a day-to-day basis.
In light of the foregoing deficiencies associated with known visual calibration techniques for display monitors that require a user to match intensities/colors, a need has been identified for a visual display monitor calibration technique that does not require a matching operation and, instead, requires the user to perform operations that are more intuitive. Two such tasks that are very common in standard user lives are (1) readability/ease of reading, where a user is asked to judge legibility of a text or general alphanumeric or iconic string and (2) comparing sizes/widths of simple adjacent/touching patterns. The terms “non-match” and “non-matching test pattern” describe cases where the user task does not involve “matching of color”, “matching of luminance”, or the like.